(1) They prefer one of the other candidates
(2) They aren't going to vote at all
(3) They prefer Nader, but don't want to "waste" their vote
Three reasons. Three bad reasons.
(1) Bush? I won't dignify that with a response unless I have to. Gore? The last time I had a party affiliation, it was to the Democrats. (And then only so I could vote for Jerry Brown in the primary.) I recognize that people might feel a loyalty to a political party for historical reasons, or family reasons, or whatever ... and while I don't mean to imply that Gore is the worst nominee the Dems ever put forward, I have to wonder: is this guy a Democrat in whatever vestigial sense of meaning that word retains? Should there be any party loyalty there? Will Bush or Gore destroy the country if elected? Probably not. Reagan was a complete tool and though we were nearly driven into the ground by his insane economic theory and demented foreign policy, we still have roads, electricity and didn't get nuked off the planet, so we can probably survive either of the two current big party puddin'heads. Even if you don't like Nader, do you think he could really do more harm as well-meaning public servant not beholden to corporate America than two politicos whose greatest desire seems to be to throw our tax dollars into the already overstuffed coffers of the rich in return for ... ? (We'll probably never know what.)
(2) The only reason not to vote is the feeling that voting only feeds "the machine," that voting implies acceptance of the current system, that something other than voting (revolution) is needed to set things right -- but there is no revolution. There could be reform though. We could elect Nader.
(3) When did voting become a game? It's not the Kentucky Derby; you don't get paid if you bet the perfecta 1-2. It's ridiculous that a vote for a candidate you actually believe in could be described as wasted. The whole gamesmanship thing with it's air of realpolitik, which is so much bullshit, that's what's making it so easy for the big money parties to prevent the development of a strong third party. There's this: "If I don't vote for Gore, then Bush might win!" For one thing, either your vote matters or it doesn't, and if you think it does and you don't vote for the candidate you think is the best, then you are just as much a tool as G.W. The other thing is, if you vote for Gore to keep Bush from winning, you moron, you just helped elect another Republicrat/Demopublican.
I'd rather Gore than Bush. But I'd rather Nader than either of them, that's why I'm going to vote for him. If my vote is the one that costs Gore the election, as if that were a sensible way to talk about an election, then so be it.
